The demand for high capacity rechargeable electrochemical cells is strong. Many areas of application, such as aerospace, medical devices, portable electronics, and automotive, require high gravimetric and/or volumetric capacity cells. Lithium ion technology represents a significant improvement in this regard. However, to date, application of this technology has been primarily limited to graphite negative electrodes, and graphite has a theoretical capacity of only about 372 mAh/g during lithiation.
Silicon, germanium, tin, and many other materials are attractive insertion negative active materials because of their high electrochemical capacity. For example, the theoretical capacity of silicon during lithiation has been estimated at about 4,200 mAh/g. Yet many of these materials have not been widely adopted in commercial cells and batteries. One reason is the substantial change in volume these materials undergo during cycling. For example, silicon swells by as much as 400% when charged to its theoretical capacity corresponding to Li4.4Si. Volume changes of this magnitude can cause considerable stresses in active material structures resulting in fractures and pulverization, loss of electrical connections within the electrode, and capacity fading of the cell.
It has been found that certain high capacity active materials may be kept below fracture stress levels during cycling by reducing the size of the active material structures. For example, silicon may be used in the form of nanowires that have sufficiently small cross sectional dimensions. Stress levels resulting from swelling of these small structures may stay be below the fracture limit for silicon. However, small structures have a large corresponding surfaces areas (per unit volume) exposed to the electrolyte. When a Solid Electrolyte Interphase (SEI) layer forms on this surface, substantial amounts of lithium are consumed and trapped in the layer making that lithium unavailable for cycling/charge carrying functions.
Overall, there is a need for improved applications of high capacity active materials in battery electrodes that minimize the drawbacks described above.